Method of improving impact resistance of mild steel



Nov. 6, 1962 s. B. HUDAK ETAL 3,062,693

METHOD OF IMPROVING IMPACT RESISTANCE OF MILD STEEL y Alarney Nov. 6, 1962 s. B. HUDAK ETAL METHOD OF IMPROVING IMPACT RESISTANCE OF MILD STEEL Filed July 2, 1959 8 Sheets-Sheet 2 oo. om o@ o oN o oN- o n om o l o@ om mm mm 3.68 m2 3.68 m2 l o9 MSE: Q ad: L om@ S4( $501 N Qd: om@ o Q09 20E E :Qzm com. 20E E zw 1 ooi s ow. l om. I oom SGNflOd .LOOd CIBQHOSGV SHBNS W/LL/M C. LESLIE and HES/VLD L. IWC/(ETT 5y Aitor/rey NOV. 6, 1962 s, B, HUDAK ETAL 3,062,693

METHOD OF IMPROVING IMPACT RESISTANCE OF MILD STEEL STEPHEN B, HUD/IK, W/LLJAM C. LESL/Eand HEGl/VALD L. RO/(ETT Mid/5% Alomey NUV- 6, 1962 s. B. HUDAK ETAI. 3,062,693

METHOD OF IMPROVING IMPACT RESISTANCE OF MLD STEEL Filed July 2, 1959 a sheets-sheet 4 DPH |05 (ALUMINUM KILLED, O.IO/o C) QUENCHED FROM ISOOF T0 |350 F, HELD I0 MINUTES, AIR COOLED TEMPERATURE, F V-NOTCH CHARPY IMPACT RESISTANCE OF STEEL C DPH |04 Fls. 4

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W/LL/AM C. ESL/E and REG/NALD L. R/C/(ETT By 91%/ )y Afarney Nov. 6, 1962 s. B. HUDAK ETAL METHOD OF' IMPROVING IMPACT RESISTANCE OF' MILD STEEL 8 Sheets-Sheet 8 Filed July 2, 1959 GN H .m55 moz/CBM". 55;: KEES 1302i .9... odnmasm Oooo 830Mo soNnod loos Gaaaosav Sama /NI/ENTORS STEPHEN B. HUD/1K, W/LL/AM C. ESL/E and REG/NALD L. RIC/(ETT United States 3,062,693- METHOD F IMPROVING IMPACT RESISTANCE 0F MILD STEEL Stephen B. Hudak, Pleasant Hills, and William C. Leslie and Reginald L. Ricltett, Franklin Township, Westmoreland County, Pa., assignors to United States Steel Corporation, a corporation ofNew Jersey Filed July 2, 1959, Ser. No. 824,590 9 Claims. (Cl. 148-134) This invention relates to a method of improving the impact resistance of mild steel and more particularly to a method of lowering the notch impact transition temperature of such steel.

Brittle fractures in large steel structures, such as ships, storage tanks, bridges and the like have caused large and unexpected losses and have been the subject of intensive investigation of both users and manufactures of mild steel. These investigations have shown that the notch impact properties of mild steels can be improved by deoxidizing with aluminum but for various reasons it is not `desirable to use such killed steels for many purposes. It has also been determined that notch impact properties can be improved by increasing the manganese/ carbon ratio which however considerably increases the steel cost.

It is accordingly an object of the present invention to provide an improved and efficient method of lowering the notch impact transition temperature of mild steel.

It is another object of this invention to provide a method of lowering the notch impact transition temperature of both killed and rimmed mild steels.

The foregoing and further objects will be apparent `when read in conjunction with the attached drawings, wherein:

FIGURES 1 through 7 are graphs with accompanying photomicrographs showing the structure and notch impact properties of various steels after treatment in accordance with the legends thereon; and

FIGURE 8 is similar to FIGURES 1 through 7 except that no photomicrographs are included therein.

In accordance with the teachings of our invention, the notch impact properties of mild steel can be markedly improved by a simple heat treatment. Both aluminum killed and rimmed steels are susceptible to improvement but the treatment is more elective for killed steels. The treatment has little or no effect on the microstructure as such. The treatment of our invention is eiective for mild steels containing more than .04% and less than .15% carbon. A mild steel as understood herein is a low carbon steel containing no alloying ingredients other than manganese, silicon or aluminum and residual amounts of other elements. The maximum of the two elements mentioned are 1.5% manganese and .50% silicon.

We have discovered that the impact resistance of rimmed or aluminum killed steels containing less than about 0.15% and preferably over about .04% carbon is improved by a heat treatment in the range between the Ael and about 1400 F., i.e. in the lower part of the alpha plus gamma eld. The advantage gained does not depend upon the previous hot rolling or prior austenitizing temperature, which can be in the range 1600 to 2200 F. Furthermore, it `does not depend upon the procedure used to attain the heat-treating temperature. The steel either may be air cooled from the austenitizing or hot-rolling temperature and then reheated to the heat-treating temperature, or it may be cooled directly from the austenitizing or hot-rolling temperature to the heat-treating temperature. Following the heat treatment, the steel may be air cooled. The advantage gained depends upon the length of the treatment in the alpha plus gamma region, up to about 24 hours.

3,062,693 Patented Nov. 6, 1962 Reference will be made to the following steels in ill-ustrating the teachings of our invention:

Mn P S Si Al As shown in FIGURE l, the V-notch Charpy impact transition temperature of steel A, a rimmed 0.10% carbon steel was found to be about F. after air cooling from 2100 F. When this steel was heated to 2100 F., quenched to l350 F., held 24 hours, then air cooled, the V -notch Charpy impact transition temperature Iwas reduced to about 55 F. In this and the other figures, the transition temperature is taken as the temperature at which the energy absorbed is half the diterence between the maximum and minimum values observed. All photomicrolgraphs are at a magnification of 100 diameters.

As shown in FIGURE 2, steel B, a steel similar to steel A, was heated to 1600D F., quenched to 1350 F., held 10 minutes, then air cooled reducing the V-notch Charpy transition temperature to about 55 F. When held 24 hours at 1350 F., the transition temperature was further reduced to about 20 F.

FIGURE 3 shows that the V-notch Charpy impact transition temperature for steel C, an aluminum-killed, 0.10% carbon steel, was about F. after air cooling from 2l00 F., about 45 F. after holding 2 hours at l350 F., and about 18 F. after holding 24 hours at l350 F. As shown by the photomicrographs of FIGURE 3, little change in microstructure or hardness accompanied this change in 'impact resistance.

The impact transition temperature of steel C referred to in FIGURE 3 was about 14 F. after austenitizing at 1600 F., quenching to 1350" F., holding 100 minutes, and air cooling. As shown in FIGURE 4, increasing the length of time at 1350 F. to 24 hours changed the impact transition temperature to 62 F. without signiiicantly changing hardness or grain size.

The impact transition temperature of steel D, a 0.10% carbon aluminum-killed steel, normalized at 1650 F. was about 7 F. As is shown in FIGURE 5, reheating to 1350 F. and holding 24 hours caused the transition temperature to drop to -45 F. A similar, although smaller, effect was noted for a 0.10% carbon rimmed steel.

In both rimmed, steel B, and killed, steel D, 0.10% carbon steels, the notch impact behavior was not improved by quenching to l450 F. and holding for various periods as shown in FIGURE 6. When the steels were quenched to 1300 F. and held, the impact properties deteriorated with time due to increase of ferrite grain size and spheroidization of carbides as shown in FIGURE 7. To prevent such microstructural changes and the resulting deterioration of impact properties, it is necessary for the lower limit of the heat-treating range to be above the Ael temperature, which is about l330 F. in the 0.10% carbon steels used. The upper limit is about 1400 F. A minimum time of about 10 minutes is required and the improvement increases with increasing time, the practical limit being about 24 hours.

In steels containing 0.16% carbon, or more, no improvement in impact properties was produced by reheating to the alpha plus gamma region after normalizing, or by quenching to this temperature range after austenitizing. This is shown for steel H in FIGURE 8. None of steels E through I responded favorably to the treatment of the in the appended claims.

We claim:

1. A method of lowering the notch impact transition temperature of mild steel containing less than .15% carbon comprising quenching the steel from an austenitizing ternperature to above the Ael temperature thereof and below about 1400 F., holding within such range for at least minutes and air cooling.

2. A method of lowering the notch impact transition temperature of mild steel containing less than .15 carbon comprising quenching the steel from an austenitizing temperature to above the Ael temperature thereof and below about 1400D F., holding within such range for between 10 minutes and about 24 hours and air cooling.

3. A method of lowering the notch impact transition temperature of mild steel containing less than .15% carbon comprising quenching the steel from an austenitizing temperature to about 1350 F., holding at such temperature for at least about 10 minutes and air cooling.

4. A method of lowering the notch impact transition temperature of mild steel containing less than .15 carbon comprising quenching the steel from an austenitizing ternperature to about 1350 F., holding at such temperature for between 100 minutes and about 24 hours and air cooling.

5. A method of lowering the notch impact transition temperature of mild steel containing less than .15 carbon comprising heating the steel to above the Ael temperature thereof and below about 1400 F., holding within such range for at least 10 minutes and cooling said steel from i said temperature without producing any substantial change in hardness or microstructure.

6. A method of lowering the notch impact transition temperature of mild steel containing less than .15% carbon comprising heating the steel to above the Ael temperature thereof and below about 1400 F., and holding within such range `for between 10 minutes and about 24 hours and cooling said steel from said temperature without producing any substantial change in hardness or microstructure.

7. A method of lowering the notch impact transition temperature of mild steel containing less than .15 carbon comprising heating the steel to about 1350" F., holding at such temperature for at least about 10 minutes and cooling said steel from said temperature without producing any substantial Vchange in hardness or microstructure.

8. A method of lowering the notch impact transition temperature of mild steel containing less than .15 carbon comprising heating the steel to about 1350 F., holding at such temperature for between 10 minutes and 24 hours and cooling said steel from said temperature without producing any substantial change in hardness or microstructure.

9. A method of lowering the notch impact transition temperature of aluminum-killed mild steel containing about .10% carbon comprising quenching the steel from about 1600 F. to about 1350 F., holding at such temperature for at least minutes and cooling said steel from said temperature without producing any substantial change in hardness or microstructure.

References Cited in the le of this patent Stoughton et al.: Engineering Metallurgy, copyright 1953 lby McGraw-Hill Book Co., pages 174 and 189 relied upon. Lib. of Congress call number 53-5559.

Iron and Steel Institute 1920, No. 11; vol. CII, Honda et al., p. 266 relied upon.

UNITED sTATEsPATENT OFFICE CERTIFICATE 0F CORRECTION Patent No., 3,062,693 November 6, 1962 Stephen B., Hudak et al.,

Column 4, line .6, 'after "14000 F ,N strike out y"anoV line 34, for "pages 174 and 189" read pages 17, 174, 175

Signed and sealed this 16th .day of April 1963o (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents 

1. A METHOD OF LOWERING THE NOTCH IMPACT TRANSITION TEMPERATURE OF MILD STEEL CONTAINING LESS THAN .15% CARBON COMPRISING QUENCHING THE STEEL FROM AN AUSTENITIZING TEMPERATURE TO ABOVE THE AE1 TEMPERATURE THEREOF AND BELOW ABOUT 1400*F., HOLDING WITHIN SUCH RANGE FOR AT LEAST 10 MINUTES AND AIR COOLING. 